This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A307546 #18 Nov 30 2022 10:28:34 %S A307546 7,14,15,20,26,28,29,36,45,48,66,70,89,98,104,115,122,126,142,152,157, %T A307546 161,164,167,177,182,186,191,194,199,202,205,216,218,219,244,264,279, %U A307546 283,295,297,299,324,329,336,342,362,408,416,423,430,440,457,498,500 %N A307546 Prime indices of A195685. %C A307546 Since only the indices are listed, this sequence is more compact than A195685. As described there, all elements of the triples (2*prime(a(n))-1, 2*prime(a(n)), 2*prime(a(n))+1) are products of exactly two distinct primes. Such values are called "squarefree semiprimes" in A006881. %H A307546 Alois P. Heinz, <a href="/A307546/b307546.txt">Table of n, a(n) for n = 1..10000</a> %F A307546 a(n) = A000720(A195685(n)). %F A307546 A195685(n) = A000040(a(n)). %e A307546 A000040(a(1)) = 17 = A195685(1). %e A307546 A000040(a(5)) = A000040(26) = 101 = A195685(5). %p A307546 with(numtheory): %p A307546 q:= n-> (p-> tau(2*p-1)=4 and tau(2*p+1)=4)(ithprime(n)): %p A307546 select(q, [$1..500])[]; # _Alois P. Heinz_, Apr 18 2019 %o A307546 (PARI) isok(k) = my(p=prime(k)); (numdiv(2*p-1) == 4) && (numdiv(2*p+1) == 4); \\ _Michel Marcus_, Nov 30 2022 %Y A307546 Cf. A000005, A000040, A000720, A006881, A195685. %K A307546 nonn %O A307546 1,1 %A A307546 _Peter Dolland_, Apr 14 2019